An image forming apparatus including an automatic color selection (ACS) function for determining whether an image read from a document is treated as a color image or a monochrome image is known. Such an image forming apparatus determines whether each pixel of image data of a page of the document is a color pixel and when the total number of pixels, or a ratio of color pixels to total pixels, is equal to or greater than a predetermined value, the image is treated as a color image. In such an image forming apparatus, it is necessary to set a reference value for determining whether an image is a color image or a monochrome image. If the color/monochrome image is not correctly determined by the ACS function, a user will have to adjust the reference value by trial and error. Therefore, adjustment(s) requiring user/operator intervention is necessary.